5 Chapter 1 Getting Started With Erlang 1.1 Introduction Introduction This is a kick start tutorial to get you started with Erlang. Everything here is true, but only part of the truth. For example, I'll only tell you the simplest form of the syntax, not all esoteric forms. Where I've greatly oversimplified things I'll write *manual* which means there is lots more information to be found in the Erlang book or in the Erlang Reference Manual. I also assume that this isn't the first time you have touched a computer and you have a basic idea about how they are programmed. Don't worry, I won't assume you're a wizard programmer Things Left Out In particular the following has been omitted: References Local error handling (catch/throw) Single direction links (monitor) Handling of binary data (binaries / bit syntax) List comprehensions How to communicate with the outside world and/or software written in other languages (ports). There is however a separate tutorial for this, Interoperability Tutorial Very few of the Erlang libraries have been touched on (for example file handling) OTP has been totally skipped and in consequence the Mnesia database has been skipped. Hash tables for Erlang terms (ETS) Changing code in running systems Getting Started with Erlang 1

6 Chapter 1: Getting Started With Erlang 1.2 Sequential Programming The Erlang Shell Most operating systems have a command interpreter or shell, Unix and Linux have many, Windows has the Command Prompt. Erlang has its own shell where you can directly write bits of Erlang code and evaluate (run) them to see what happens (*manual*). Start the Erlang shell (in Linux or UNIX) by starting a shell or command interpreter in your operating system and typing erl, you will see something like this. % erl Erlang (BEAM) emulator version 5.2 [source] [hipe] Eshell V5.2 1> (abort with ^G) Now type in as shown below. 1> > In Windows, the shell is started by double-clicking on the Erlang shell icon. You'll notice that the Erlang shell has numbered the lines that can be entered, (as 1> 2>) and that it has correctly told you that is 7! Also notice that you have to tell it you are done entering code by finishing with a full stop. and a carriage return. If you make mistakes writing things in the shell, you can delete things by using the backspace key as in most shells. There are many more editing commands in the shell (*manual*). (Note: you will find a lot of line numbers given by the shell out of sequence in this tutorial as it was written and the code tested in several sessions). Now let's try a more complex calculation. 2> ( ) * 66 / Here you can see the use of brackets and the multiplication operator * and division operator /, just as in normal arithmetic (*manual*). To shutdown the Erlang system and the Erlang shell type Control-C. You will see the following output: BREAK: (a)bort (c)ontinue (p)roc info (i)nfo (l)oaded (v)ersion (k)ill (D)b-tables (d)istribution a % Type a to leave the Erlang system. Another way to shutdown the Erlang system is by entering halt(): 2 Getting Started with Erlang

7 1.2: Sequential Programming 3> halt(). % Modules and Functions A programming language isn't much use if you can just run code from the shell. So here is a small Erlang program. Enter it into a file called tut.erl (the file name tut.erl is important, also make sure that it is in the same directory as the one where you started erl (*manual*) using a suitable text editor. If you are lucky your editor will have an Erlang mode which will make it easier for you to enter and format your code nicely (*manual*), but you can manage perfectly well without. Here's the code to enter: -module(tut). -export([double/1]). double(x) -> 2 * X. It's not hard to guess that this program doubles the value of numbers. I'll get back to the first two lines later. Let's compile the program. This can be done in your Erlang shell as shown below: 3> c(tut). fok,tutg The fok,tutg tells you that the compilation was OK. If it said error instead, you have made some mistake in the text you entered and there will also be error messages to give you some idea as to what has gone wrong so you can change what you have written and try again. Now lets run the program. 4> tut:double(10). 20 As expected double of 10 is 20. Now let's get back to the first two lines. Erlang programs are written in files. Each file contains what we call an Erlang module. The first line of code in the module tells us the name of the module (*manual*). -module(tut). This tells us that the module is called tut. Note the. at the end of the line. The files which are used to store the module must have the same name as the module but with the extension.erl. In our case the file name is tut.erl. When we use a function in another module, we use the syntax, module name:function name(arguments). So 4> tut:double(10). means call function double in module tut with argument 10. The second line: Getting Started with Erlang 3

8 Chapter 1: Getting Started With Erlang -export([double/1]). says that the module tut contains a function called double which takes one argument (X in our example) and that this function can be called from outside the module tut. More about this later. Again note the. at the end of the line. Now for a more complicated example, the factorial of a number (e.g. factorial of 4 is 4 * 3 * 2 * 1). Enter the following code in a file called tut1.erl. -module(tut1). -export([fac/1]). fac(1) -> 1; fac(n) -> N * fac(n - 1). Compile the file 5> c(tut1). fok,tut1g And now calculate the factorial of 4. 6> tut1:fac(4). 24 The first part: fac(1) -> 1; says that the factorial of 1 is 1. Note that we end this part with a ; which indicates that there is more of this function to come. The second part: fac(n) -> N * fac(n - 1). says that the factorial of N is N multiplied by the factorial of N - 1. Note that this part ends with a. saying that there are no more parts of this function. A function can have many arguments. Let's expand the module tut1 with the rather stupid function to multiply two numbers: 4 Getting Started with Erlang

9 1.2: Sequential Programming -module(tut1). -export([fac/1, mult/2]). fac(1) -> 1; fac(n) -> N * fac(n - 1). mult(x, Y) -> X * Y. Note that we have also had to expand the -export line with the information that there is another function mult with two arguments. Compile: 7> c(tut1). fok,tut1g and try it out: 8> tut1:mult(3,4). 12 In the example above the numbers are integers and the arguments in the functions in the code, N, X, Y are called variables. Variables must start with a capital letter (*manual*). Examples of variable could be Number, ShoeSize, Age etc Atoms Atoms are another data type in Erlang. Atoms start with a small letter (*manual*), for example: charles, centimeter, inch. Atoms are simply names, nothing else. They are not like variables which can have a value. Enter the next program (file: tut2.erl) which could be useful for converting from inches to centimeters and vice versa: -module(tut2). -export([convert/2]). convert(m, inch) -> M / 2.54; convert(n, centimeter) -> N * Compile and test: Getting Started with Erlang 5

10 Chapter 1: Getting Started With Erlang 9> c(tut2). fok,tut2g 10> tut2:convert(3, inch) > tut2:convert(7, centimeter) Notice that I have introduced decimals (floating point numbers) without any explanation, but I guess you can cope with that. See what happens if I enter something other than centimeter or inch in the convert function: 13> tut2:convert(3, miles). =ERROR REPORT==== 28-May-2003::18:36:27 === Error in process <0.25.0> with exit value: ffunction clause,[ftut2,convert,[3,miles]g,ferl eval,expr,3g ** exited: ffunction clause,[ftut2,convert,[3,miles]g, ferl eval,expr,3g, ferl eval,exprs,4g, fshell,eval loop,2g]g ** The two parts of the convert function are called its clauses. Here we see that miles is not part of eitheroftheclauses.theerlangsystemcan'tmatch either of the clauses so we get an error message function clause. The above output looks rather a mess, but with a little practice, you can see from it exactly where in the code the error occurred Tuples Now the tut2 program is hardly good programming style. Consider: tut2:convert(3, inch). Does this mean that 3 is in inches? or that 3 is in centimeters and we want to convert it to inches? So Erlang has a way to group things together to make things more understandable. We call these tuples. Tuples are surrounded by f and g. So we can write finch,3g to denote 3 inches and fcentimeter,5g to denote 5 centimeters. Now let's write a new program which converts centimeters to inches and vice versa. (file tut3.erl). -module(tut3). -export([convert_length/1]). convert_length({centimeter, X}) -> {inch, X / 2.54}; convert_length({inch, Y}) -> {centimeter, Y * 2.54}. Compile and test: 6 Getting Started with Erlang

11 1.2: Sequential Programming 14> c(tut3). fok,tut3g 15> tut3:convert length(finch, 5g). fcentimeter, g 16> tut3:convert length(tut3:convert length(finch, 5g)). finch, g Note on line 16 we convert 5 inches to centimeters and back again and reassuringly get back to the original value. I.e the argument to a function can be the result of another function. Pause for a moment and consider how line 16 (above) works. The argument we have given the function finch,5g is first matched against the first head clause of convert length i.e. convert length(fcentimeter,xg) where it can be seen that fcentimeter,xg does not match finch,5g (the head is the bit before the -> ). This having failed, we try the head of the next clause i.e. convert length(finch,yg), this matches and Y get the value 5. We have shown tuples with two parts above, but tuples can have as many parts as we want and contain any valid Erlang term. For example, to represent the temperature of various cities of the world we could write {moscow, {c, -10}} {cape_town, {f, 70}} {paris, {f, 28}} Tuples have a fixed number of things in them. We call each thing in a tuple an element. So in the tuple fmoscow,fc,-10gg,element 1 is moscow and element 2 is fc,-10g. I have chosen c meaning Centigrade (or Celsius) and f meaning Fahrenheit Lists Whereas tuples group things together, we also want to be able to represent lists of things. Lists in Erlang are surrounded by [ and ]. For example a list of the temperatures of various cities in the world could be: [{moscow, {c, -10}}, {cape_town, {f, 70}}, {stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}] Note that this list was so long that it didn't fit on one line. This doesn't matter, Erlang allows line breaks at all sensible places but not, for example, in the middle of atoms, integers etc. A very useful way of looking at parts of lists, is by using. This is best explained by an example using the shell. 18> [First TheRest] = [1,2,3,4,5]. [1,2,3,4,5] 19> First. 1 20> TheRest. [2,3,4,5] Getting Started with Erlang 7

12 Chapter 1: Getting Started With Erlang We use to separate the first elements of the list from the rest of the list. (First has got value 1 and TheRest value [2,3,4,5]). Another example: 21> [E1, E2 R] = [1,2,3,4,5,6,7]. [1,2,3,4,5,6,7] 22> E > E > R. [3,4,5,6,7] Here we see the use of to get the first two elements from the list. Of course if we try to get more elements from the list than there are elements in the list we will get an error. Note also the special case of the list with no elements []. 25> [A, B C] = [1, 2]. [1,2] 26> A. 1 27> B. 2 28> C. [] In all the examples above, I have been using new variable names, not reusing the old ones: First, TheRest, E1, E2, R, A, B, C. The reason for this is that a variable can only be given a value once in its context (scope). I'll get back to this later, it isn't so peculiar as it sounds! The following example shows how we find the length of a list: -module(tut4). -export([list_length/1]). list_length([]) -> 0; list_length([first Rest]) -> 1 + list_length(rest). Compile (file tut4.erl) and test: 29> c(tut4). fok,tut4g 30> tut4:list length([1,2,3,4,5,6,7]). 7 Explanation: 8 Getting Started with Erlang

13 1.2: Sequential Programming list_length([]) -> 0; The length of an empty list is obviously 0. list_length([first Rest]) -> 1 + list_length(rest). The length of a list with the first element First and the remaining elements Rest is 1 + the length of Rest. (Advanced readers only: This is not tail recursive, there is a better way to write this function). In general we can say we use tuples where we would use records or structs in other languages and we use lists when we want to represent things which have varying sizes, (i.e. where we would use linked lists in other languages). Erlang does not have a string date type, instead strings can be represented by lists of ASCII characters. So the list [97,98,99] is equivalent to abc. The Erlang shell is clever and guesses the what sort of list we mean and outputs it in what it thinks is the most appropriate form, for example: 31> [97,98,99]. "abc" Standard Modules and Manual Pages Erlang has a lot of standard modules to help you do things. For example, the module io contains a lot of functions to help you do formatted input/output. To look up information about standard modules, the command erl -man can be used at the operating shell or command prompt (i.e. at the same place as that where you started erl). Try the operating system shell command: % erl -man io ERLANG MODULE DEFINITION io(3) MODULE io - Standard I/O Server Interface Functions DESCRIPTION This module provides an interface to standard Erlang IO servers. The output functions all return ok if they are suc-... If this doesn't work on your system, the documentation is included as HTML in the Erlang/OTP release, or you can read the documentation as HTML or download it as PDF from either of the sites (commercial Erlang) or (open source), for example for release R9B: Getting Started with Erlang 9

14 Chapter 1: Getting Started With Erlang Writing Output to a Terminal It's nice to be able to do formatted output in these example, so the next example shows a simple way to use to use the io:format function. Of course, just like all other exported functions, you can test the io:format function in the shell: 32> io:format("hello world~n", []). hello world ok 33> io:format("this outputs one Erlang term: ~w~n", [hello]). this outputs one Erlang term: hello ok 34> io:format("this outputs two Erlang terms: ~w~w~n", [hello, world]). this outputs two Erlang terms: helloworld ok 35> io:format("this outputs two Erlang terms: ~w ~w~n", [hello, world]). this outputs two Erlang terms: hello world ok The function format/2 (i.e. format with two arguments) takes two lists. The first one is nearly always a list written between. This list is printed out as it stands, except that each ~w is replaced by a term taken in order from the second list. Each ~n is replaced by a new line. The io:format/2 function itself returns the atom ok if everything goes as planned. Like other functions in Erlang, it crashes if an error occurs. This is not a fault in Erlang, it is a deliberate policy. Erlang has sophisticated mechanisms to handle errors which we will show later. As an exercise, try to make io:format crash, it shouldn't be difficult. But notice that although io:format crashes, the Erlang shell itself does not crash A Larger Example Now for a larger example to consolidate what we have learnt so far. Assume we have a list of temperature readings from a number of cities in the world. Some of them are in Celsius (Centigrade) and some in Fahrenheit (as in the previous list). First let's convert them all to Celsius, then let's print out the data neatly. %% This module is in file tut5.erl -module(tut5). -export([format_temps/1]). %% Only this function is exported format_temps([])-> % No output for an empty list ok; format_temps([city Rest]) -> print_temp(convert_to_celsius(city)), format_temps(rest). convert_to_celsius({name, {c, Temp}}) -> {Name, {c, Temp}}; convert_to_celsius({name, {f, Temp}}) -> {Name, {c, (Temp - 32) * 5 / 9}}. % No conversion needed % Do the conversion 10 Getting Started with Erlang

15 1.2: Sequential Programming print_temp({name, {c, Temp}}) -> io:format("~-15w ~w c~n", [Name, Temp]). 36> c(tut5). fok,tut5g 37> tut5:format temps([fmoscow, fc, -10gg, fcape town, ff, 70gg, fstockholm, fc, -4gg, fparis, ff, 28gg, flondon, ff, 36gg]). moscow -10 c cape town c stockholm -4 c paris c london c ok Before we look at how this program works, notice that we have added a few comments to the code. A comment starts with a % character and goes on to the end of the line. Note as well that the -export([format temps/1]). line only includes the function format temps/1, the other functions are local functions, i.e. they are not visible from outside the module tut5. Note as well that when testing the program from the shell, I had to spread the input over two lines as the line was too long. When we call format temps the first time, City gets the value fmoscow,fc,-10gg and Rest is the rest of the list. So we call the function print temp(convert to celsius(fmoscow,fc,-10gg)). Here we see a function call as convert to celsius(fmoscow,fc,-10gg) as the argument to the function print temp. Whenwenest function calls like this we execute (evaluate) them from the inside out. I.e. we first evaluate convert to celsius(fmoscow,fc,-10gg) which gives the value fmoscow,fc,-10gg as the temperature is already in Celsius and then we evaluate print temp(fmoscow,fc,-10gg). The function convert to celsius works in a similar way to the convert length function in the previous example. print temp simply calls io:format in a similar way to what has been described above. Note that ~-15w says to print the term with a field length (width) of 15 and left justify it. (*manual*). Now we call format temps(rest) with the rest of the list as an argument. This way of doing things is similar to the loop constructs in other languages. (Yes, this is recursion, but don't let that worry you). So the same format temps function is called again, this time City gets the value fcape town,ff,70gg and we repeat the same procedure as before. We go on doing this until the list becomes empty, i.e. [], which causes the first clause format temps([]) to match. This simply returns (results in) the atom ok, so the program ends Matching, Guards and Scope of Variables It could be useful to find the maximum and minimum temperature in lists like this. Before extending the program to do this, let's look at functions for finding the maximum value of the elements in a list: -module(tut6). -export([list_max/1]). list_max([head Rest]) -> list_max(rest, Head). list_max([], Res) -> Getting Started with Erlang 11

16 Chapter 1: Getting Started With Erlang Res; list_max([head Rest], Result_so_far) when Head > Result_so_far -> list_max(rest, Head); list_max([head Rest], Result_so_far) -> list_max(rest, Result_so_far). 39> c(tut6). fok,tut6g 40> tut6:list max([1,2,3,4,5,7,4,3,2,1]). 7 First note that we have two functions here with the same name list max. However each of these takes a different number of arguments (parameters). In Erlang these are regarded as completely different functions. Where we need to distinguish between these functions we write name/arity, where name is the name of the function and arity is the number of arguments, in this case list max/1 and list max/2. This is an example where we walk through a list carrying a value with us, in this case Result so far. list max/1 simply assumes that the max value of the list is the head of the list and calls list max/2 with the rest of the list and the value of the head of the list, in the above this would be list max([2,3,4,5,7,4,3,2,1],1). If we tried to use list max/1 with an empty list or tried to use it with something which isn't a list at all, we would cause an error. Note that the Erlang philosophy is not to handle errors of this type in the function they occur, but to do so elsewhere. More about this later. In list max/2 we walk down the list and use Head instead of Result so far when Head > Result so far. when is a special word we use before the -> in the function to say that we should only use this part of the function if the test which follows is true. We call tests of this type a guard. Ifthe guard isn't true (we say the guard fails), we try the next part of the function. In this case if Head isn't greater than Result so far then it must be smaller or equal to is, so we don't need a guard on the next part of the function. Some useful operators in guards are, < less than, > greater than, == equal, >= greater or equal, <= less or equal, /= not equal. (*manual*). To change the above program to one which works out the minimum value of the element in a list, all we would need to do is to write < instead of >. (But it would be wise to change the name of the function to list min :-). Remember that I mentioned earlier that a variable could only be given a value once in its scope? In the above we see, for example, that Result so far has been given several values. This is OK since every time we call list max/2 we create a new scope and one can regard the Result so far as a completely different variable in each scope. Another way of creating and giving a variable a value is by using the match operator =. So if I write M = 5, a variable called M will be created and given the value 5. If, in the same scope I then write M = 6, I'll get an error. Try this out in the shell: 41> M = > M = 6. ** exited: ffbadmatch,6g,[ferl eval,expr,3g]g ** 43> M = M + 1. ** exited: ffbadmatch,6g,[ferl eval,expr,3g]g ** 44> N = M Getting Started with Erlang

17 1.2: Sequential Programming The use of the match operator is particularly useful for pulling apart Erlang terms and creating new ones. 45> fx, Yg = fparis, ff, 28gg. fparis,ff,28gg 46> X. paris 47> Y. ff,28g Here we see that X gets the value paris and Y ff,28g. Of course if we try to do the same again with another city, we get an error: 49> fx, Yg = flondon, ff, 36gg. ** exited: ffbadmatch,flondon,ff,36ggg,[ferl eval,expr,3g]g ** Variables can also be used to improve the readability of programs, for example, in the list max/2 function above, we could write: list_max([head Rest], Result_so_far) when Head > Result_so_far -> New_result_far = Head, list_max(rest, New_result_far); which is possibly a little clearer More About Lists Remember that the operator can be used to get the head of a list: 50> [M1 T1] = [paris, london, rome]. [paris,london,rome] 51> M1. paris 52> T1. [london,rome] The operator can also be used to add a head to a list: 53> L1 = [madrid T1]. [madrid,london,rome] 54> L1. [madrid,london,rome] Now an example of this when working with lists - reversing the order of a list: Getting Started with Erlang 13

18 Chapter 1: Getting Started With Erlang -module(tut8). -export([reverse/1]). reverse(list) -> reverse(list, []). reverse([head Rest], Reversed_List) -> reverse(rest, [Head Reversed_List]); reverse([], Reversed_List) -> Reversed_List. 56> c(tut8). fok,tut8g 57> tut8:reverse([1,2,3]). [3,2,1] Consider how Reversed List is built. It starts as [], we then successively take off the heads of the list to be reversed and add them to the the Reversed List, as shown in the following: reverse([1 2,3], []) => reverse([2,3], [1 []]) reverse([2 3], [1]) => reverse([3], [2 [1]) reverse([3 []], [2,1]) => reverse([], [3 [2,1]]) reverse([], [3,2,1]) => [3,2,1] The module lists contains a lot of functions for manipulating lists, for example for reversing them, so before you write a list manipulating function it is a good idea to check that one isn't already written for you. (*manual*). Now lets get back to the cities and temperatures, but take a more structured approach this time. First let's convert the whole list to Celsius as follows and test the function: -module(tut7). -export([format_temps/1]). format_temps(list_of_cities) -> convert_list_to_c(list_of_cities). convert_list_to_c([{name, {f, F}} Rest]) -> Converted_City = {Name, {c, (F -32)* 5 / 9}}, [Converted_City convert_list_to_c(rest)]; convert_list_to_c([city Rest]) -> [City convert_list_to_c(rest)]; 14 Getting Started with Erlang

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